Temporal and Spatial Evolution of the Asian Summer Monsoon in the Seasonal Cycle of Synoptic Fields

Abstract

The principal mode of the seasonal variation of the Asian summer monsoon (ASM) and the temporal and spatial evolution of the corresponding synoptic fields are investigated via cyclostationary EOF analysis. This study uses the 21-yr (1979?99) Xie?Arkin precipitation pentad data and National Centers for Environmental Prediction daily reanalysis data focusing on the period 21 May to 28 August, which covers the prominent life cycle of the ASM. The first mode, representing the seasonal cycle, explains about 20%?40% of the total variability in the parameters considered in the study. The pronounced feature in the present study is that the seasonal evolution of the sea level pressure anomaly contrast between the Asian continent and the surrounding oceans is the governing mechanism for the ASM evolution. The northward migration of the low pressure anomaly from the Indian Ocean and the moisture transport by a low-level westerly (Somali jet) toward India describes the evolution of the precipitation field over India, the Bay of Bengal, and the Indochina peninsula. Over the Pacific Ocean, a high pressure anomaly to the south of the precipitation band between 25° and 40°N pushes it northward, characterizing the onset and evolution of regional monsoons (mei-yu, baiu, and changma) in east Asia. It is shown that the high pressure anomaly intruding into southern China and farther west to the Bay of Bengal from mid-June to mid-July accelerates the low-level wind along the coastal line of Asia, where the pressure gradient becomes maximum. It provides a favorable condition for moisture transport toward the east Asian monsoon region. Two major moisture sources, the Indian Ocean and the western Pacific Ocean, are found during the monsoon period. Evolutions of anomalous moisture transport patterns toward India, the Indochina peninsula, and east Asia show unique characteristics from one another. While precipitation over the Indian region is persistently affected by the Indian Ocean, the Indochina peninsula is dominated by the western Pacific Ocean in the early monsoon period followed by the effect of the Indian Ocean (mid June to mid July) and the cooperation of the two sources afterward. The east Asian monsoon regions are influenced by both sources from the onset of their monsoons. After late July, the latter source affects the east Asian monsoon more due to the northward movement of a high pressure anomaly (late July) together with the development of a low pressure anomaly over the subtropical western Pacific region.